Equilibrium study of the systems of aluminium(III), gallium(III) and indium(III) with mercaptoacetate, 3-mercaptopropionate and 2-mercaptobenzoate

Abstract

Equilibrium studies were carried out by pH-potentiometry on the systems of aluminium(III), gallium(III) and indium(III) with mercaptoacetate (MerAc 2−), 3-mercaptopropionate (MerPr 2−) and 2-mercaptobenzoate (MerBe 2−). It was found that the complex-forming properties of the Al 3+ ion towards these mercaptocarboxylic acid ligands differ from those of Ga 3+ and Al 3+. Under the conditions of the study, Al 3+ forms only hydroxo complexes, while Ga 3+ and In 3+ form relatively stable complexes involving the simultaneous coordination of the carboxylate and the deprotonated mercapto group. In all cases the equilibrium systems can be described without the assumption of polynuclear complexes. The complexes Ga(MerAc) 2 and Ga(MerBe) 2 show marked stability; this was interpreted in terms of back-coordination and of interaction between the d 10 electrons of the Ga 3+ ion and the empty d orbitals of the S donor atom. Complexes of composition ML i are not formed in the Ga 3+-MerPr 2− system; this points to the importan roles of the number of atoms in the chelate ring and the higher stability of the Ga(III)-hydroxo complexes.